The study of the interactions of polycyclic aromatic hydrocarbon carcinogens with nucleic acids and with proteins in proposed. The goal of this study is to help establish the physico-chemical factors which render some polycyclic aromatic hydrocarbons effective carcinogens and sensitizers in the photodynamic effect, while other structurally similar compounds are ineffective carcinogens and display low photodynamic activity. The polycyclic aromatic hydrocarbons form complexes with nucleic acids and proteins which are dissolved in aqueous solutions at room temperature. The accessibilities and reactivities of these aromatic molecules which are located in hydrophobic regions of the host are investigated using the relatively long-lived triplet excited states of the polycyclic aromatic molecules as probes of their environment in biomacromolecule complexes. The technique of flash photolysis is used in which the aromatic hydrocarbon molecules are excited to their triplet states and the lifetimes of the latter are determined by monitoring the transient triplet-triplet absorption. The potential reactivities and accessibilities of the complexed carcinogens and allied compounds to other molecules (such reactions occur in the metabolic activation of these compounds and in the photodynamic effect) are probed by utilizing the fact that the triplet excited states are readily quenched by many different types of paramagnetic ions or molecules upon bimolecular collision or encounter.